The present invention relates to a method of operating nuclear reactors, in particular, to a method of operating a boiling-water reactor.
The boiling-water reactors have an axial void distribution so that the power distribution is skewed toward the bottom of the reactor core and the power peaking is relatively high. In order to suppress the power peaking, the control rods are inserted shallow from the bottom of the reactor core.
FIG. 1 shows in horizontal section an example of the conventional control rod pattern. Each square includes one control rod surrounded by four fuel assemblies. The numerals in squares indicate the control rod insertion ratio. That is, the axial height of the reactor core is divided into 24 unit lengths, and the numeral in a square shows that the control rod is inserted from the bottom of the reactor core to the depth indicated by that unit lengths. Therefore, the higher the numeral, the deeper the control rod is inserted. The blank square with no numeral indicates that the control rod is completely or fully withdrawn. It can be seen that the control rod pattern is very complicated. Many calculations are required for determining the control rod pattern. Furthermore there arises the problem that the power changes abruptly in the vicinity of the tip end of the control rod which is being inserted shallow.
In order to overcome the above problems, Japanese Patent Application Nos. 51-116268 and 51-115269 disclose the reactor in which the reactor core is divided into two regions at the axial midpoint between the top and bottom thereof so that the infinite multiplication factor may be higher in the upper region than the lower region and consequently the axial power distribution may be flattened. FIG. 2 shows, by curve 1, the average axial power distribution in such a core when the control rods are fully withdrawn. It can be seen that, as compared with the distribution curve 2 of the conventional reactor core, the power distribution is remarkably flattened. According to the above inventions, it is only required to flatten the power distribution in the core in the radial direction thereof with the control rods. That is, the power distribution is only dependent upon the pattern of control rods which are inserted deep. However, even if the above reactor is operated with the control rod pattern constituted merely by withdrawing shallow control rods from the conventional control rod pattern shown in FIG. 1, the satisfactory flattening of the output distribution cannot be attained because the conventional control rod pattern is intended to flatten the power distribution with the combination of the control rods which are inserted deep and shallow. As a result, the advantages of the above reactor core cannot be fully substantiated.